1. Field of the Invention
The present invention relates to an image forming apparatus adapted to detect amount of remaining toner.
2. Description of the Related Art
An electrostatic capacitance detection method described in Japanese Patent Application Laid-Open No. 8-44184 and a light transmission method described in Japanese Patent Application Laid-Open No. 2000-131936 are used as mechanisms for detecting amount of toner remaining in an electrophotographic image forming apparatus. Detection of remaining amount of toner according to the electrostatic capacitance detection method is used as a technique ideal for application primarily to an image forming apparatus having an AC bias power supply for non-contact development, such as a monochrome image forming apparatus. Detection of remaining amount of toner according to the light transmission method is used as a technique ideal for application primarily to an image forming apparatus, such as a color image forming apparatus, which performs contact development of non-magnetic toner by DC bias and which does not have an AC bias power supply for development.
FIG. 16 illustrates the configuration of an image forming apparatus as one example of an electrophotographic image forming apparatus. In the image forming apparatus shown in FIG. 16, a charging roller 2 serving as charging means uniformly charges the surface of a photosensitive drum 1 serving as an image carrier. An exposure unit 3 then subjects the surface of the photosensitive drum 1 to exposure scanning based upon image information, thereby forming an electrostatic latent image. Next, the electrostatic latent image is visualized by toner (developer) contained in a developing unit 4, whereby a toner image, i.e., developer image, is formed on the photosensitive drum 1. Printing paper 16 contained in a paper-feed cassette 8 is fed into the image forming apparatus by a feed roller 9 and the toner image on the photosensitive drum 1 is transferred onto the printing paper 16 by a transfer roller 11 serving as transfer means. The unfixed toner image on the printing paper 16 is fixed on the printing paper 16 by a fixing unit 12 using heat and pressure.
FIG. 17 illustrates the configuration of a developing unit as one example of the developing unit 4 used in the image forming apparatus. In the developing unit 4 shown in FIG. 17, toner 20 is stored in a toner container 100 serving as a toner accommodating unit. By rotating a stirring bar 103, the toner 20 is conveyed to the opening of the toner container 100 located at a developing position where the developing unit 4 and photosensitive drum 1 oppose each other. The opening of the toner container 100 has a developing roller 101, which serves as a developer carrier, for supplying the toner 20 to the electrostatic latent image that has been formed on the photosensitive drum 1. The toner 20 is supplied to the developing roller 101, and toner 20 that has not contributed to development of the electrostatic latent image formed on the photosensitive drum 1 and has been returned to the developing unit 4 is scraped off the developing roller 101 by an RS roller 102, which is placed in contact with the developing roller 101. The developing unit 4 is constructed as a process cartridge removably installed in the main body of the image forming apparatus and is in wide use. Since the image formation described above is carried out using the toner 20, it becomes necessary to prompt the user to replenish the toner 20 when only a small amount of toner is left. Accordingly, the image forming apparatus has a mechanism for detecting remaining amount of toner. This mechanism detects the amount of toner 20 remaining inside the process cartridge, i.e., inside the developing unit 4. A mechanism and method for detecting remaining amount of toner based upon two methods, namely electrostatic capacitance detection and light transmission, will be described below.
FIG. 18 illustrates the configuration of a mechanism for detecting remaining amount of toner based upon electrostatic capacitance detection. An antenna 104 in FIG. 18 is an electrode for detecting remaining amount of toner and is disposed in parallel with the developing roller 101 and spaced a prescribed distance away from the roller. The antenna 104 possesses electrostatic capacitance between itself and the developing roller 101. As the toner 20 inside the toner container 100 is consumed, the toner between the developing roller 101 and antenna 104 decreases. As a result, the dielectric constant between the developing roller 101 and antenna 104 decreases and so does the electrostatic capacitance. By sensing the change in electrostatic capacitance, the amount of toner 20 remaining in the toner container 100 can be detected. In other words, when a prescribed AC voltage is applied to the developing roller 101 by an AC developing high-voltage power supply 105, an AC current value Il conforming to the electrostatic capacitance of an equivalent capacitor 106 formed between the developing roller 101 and antenna 104 is obtained. The AC current value Il is proportional to the product of the frequency and amplitude of the AC developing high-voltage power supply 105 and electrostatic capacitance of the equivalent capacitor 106. The AC current value Il is rectified by a rectifying circuit constructed by diodes 201, 202, resistor 203 and capacitor 204, converted to a voltage value V1 and input to the inverting input terminal of a comparator 108. Similarly, when a prescribed AC voltage is applied to a reference capacitor 107 by the AC developing high-voltage power supply 105, an AC current value 12 conforming to the electrostatic capacitance of the reference capacitor 107 is obtained. The AC current value I2 is rectified by a rectifying circuit constructed by diodes 205, 206, resistor 207 and capacitor 208, converted to a reference voltage value V2 for detecting remaining amount of toner and input to a non-inverting input terminal of the comparator 108. The result of comparison by the comparator 108, i.e., a detection result 110 conforming to the remaining amount of toner, is sent to a controller (not shown) within the image forming apparatus. Based upon the detection result 110, whether the amount of toner 20 remaining inside the toner container 100 is less than a prescribed amount of toner can be detected.
Further, FIG. 19 illustrates the configuration of a mechanism in which the comparison circuit based upon the comparator 108 of FIG. 18 is replaced by an integrating circuit formed by an operational amplifier 109, resistors 209, 210 and capacitor 211. Here the error between the voltages V1, V2 applied to inverting and non-inverting input terminals of the operational amplifier 109 is amplified and detected as the detection result 110. That is, the amount of toner remaining inside the toner container 100 can be detected successively as an analog quantity (see Japanese Patent Application Laid-Open No. 8-44184).
FIG. 20 illustrates the configuration of a mechanism for detecting remaining amount of toner based upon the light transmission method. As shown in FIG. 20, the toner container 100 is provided with transparent windows 401, 402 through which light is transmitted. A photodiode 403 serving as a light-emitting member is provided. A phototransistor 404 serving as a light-receiving member is placed at a position where it will intercept light that has passed through the windows 401, 402 when the light is emitted from the photodiode 403. That is, an optical circuit is formed in such a manner that light emitted from the photodiode 403 passes through the interior of the toner container 100 and is received at the phototransistor 404 situated opposite the photodiode 403. As the toner 20 inside the toner container 100 is consumed, the time it takes for the optical circuit to be formed when the toner is stirred inside the toner container 100 by rotating the stirring bar 103, i.e., the time it takes for light to be transmitted, lengthens. The amount of toner 20 remaining inside the toner container 100 can be detected by sensing a change in the light transmission time. In other words, when light emitted from the photodiode 403 has been sensed, the time it takes for a pulsed voltage waveform that is output from the phototransistor 404 to exceed a preset value is sensed. By sending the sensed time to a controller (not shown), it is possible to sense whether the amount of toner 20 remaining inside the toner container 100 has fallen below a prescribed amount or to sense the remaining amount of toner 20 inside the toner container 100 successively as an analog value (see Japanese Patent Application Laid-Open No. 2000-131936).
Thus, detection of the amount of remaining toner by the electrostatic capacitance detection method is used as a technique ideal for application to a monochrome image forming apparatus having an AC bias power supply for development. Further, the detection of amount of remaining toner by the light transmission method is ideal as a technique for application to a color image forming apparatus that does not use an AC bias power supply for development.
Recently, a developing unit 4 from which the stirring bar 103 has been removed from within the toner container 100 has been proposed for the purpose of reducing the size, weight and cost of the developing unit 4, as illustrated in FIG. 21. However, since the developing unit 4 does not have means for stirring the toner 20 inside the toner container 100, it is difficult to employ the light transmission method in order to detect the remaining amount of toner. In this developing unit 4, therefore, the selection made is detection of remaining amount of toner by the electrostatic capacitance detection method that senses a change in the electrostatic capacitance of the equivalent capacitor 106 formed between the developing roller 101 and RS roller 102.
It should be noted that the RS roller 102 is a member for removing toner from and supplying it to the developing roller 101. In this case, it is required that a color image forming apparatus be provided anew with a special-purpose AC power supply 111 for applying AC voltage to either the developing roller 101 or RS roller 102 only at the time of detection of remaining amount of toner in order to detect the remaining amount of toner by the electrostatic capacitance detection method. Providing the AC power supply 111 raises the cost of the image forming apparatus proper and is a factor that impedes a reduction in the cost of the developing unit 4.